Mode filters

Abstract

The invention relates to mode filters for millimetric waves. The mode filter provided includes a circularly sectioned waveguide divided into a number of equal sectorial channels. Dielectric mode perturbing material is provided in all but one of the channels. This material extends in the direction of the H-field so as not to interfere with a wanted mode of a propagated signal but to perturb the current lines of an unwanted TE On mode so that an -unwanted TE On mode in the channel containing the dielectric material undergoes a πradian phase shift relative to the TE On mode in the remaining channel which remaining channel has a conductive sectorial ridge of the same cross section as the sectorial channels and extending in the direction of the H-field the ridge being of such a radius that differential phase shifts produced by the perturbation effect of the mode perturbing material of the wanted mode cut-off frequency are substantially reduced.

We claim: 
     
       1. A mode filter for millimetric waves including a circularly sectioned waveguide divided into n equal sectorial channels, mode perturbing means in said channels extending in the direction of the H-field so as not to substantially interfere with a wanted mode of a signal, provided in operation, but to perturb the current lines of a predetermined unwanted mode, and a conductive sectorial ridge having the same cross-section as the sectorial channels which ridge also extends in the direction of the H-field and is positioned along the longitudinal axis of the circularly sectioned waveguide in at least one of the sectorial channels, said ridge being of such a radius that differential phase shifts produced by the perturbation effect of the mode perturbing means of the wanted mode cut-off frequency are substantially reduced. 
     
     
       2. A filter as claimed in claim 1 and wherein the mode perturbation means is a dielectric material provided in (n-1) of the channels such that an unwanted TE On .sup..increment. mode in said (n-1) channel or channels undergoes a π radian phase shift relative the TE On .sup..increment. mode in the remaining channel, and the ridge is positioned in said remaining channel. 
     
     
       3. A filter as claimed in claim 1 wherein n is greater than 2 and the dielectric material has different lengths in the channels said ridge being positioned in channels other than said remaining channel in dependence upon the differential phase shifts of the wanted mode in the various channels. 
     
     
       4. A mode filter for millimetric waves comprising, in combination: a length of circular waveguide adapted to be inserted into a waveguide in which a wanted TE mode and an unwanted TE mode are propogated;   wall means dividing said length of waveguide into at least two equal sectorial channels and including perturbation means in the form of a pair of parallel, longitudinal slots in the wall means defining one of said channels and dielectric means in said slots for preferentially affecting the current lines of said unwanted mode in said one channel, said slots being equally spaced on opposite sides of the longitudinal axis of the waveguide and having a width and depth which marginally increases the cut-off frequency of said wanted mode and tends to decrease the cut-off frequency of said unwanted mode; and   a conductive sectorial ridge on said wall means within the remaining one of said channels, said ridge being concentrically positioned along the longitudinal axis of said length of waveguide and having a radius less than the radius of said length of waveguide such that the cut-off frequency of said wanted mode is increased while the cut-off frequency of the unwanted mode likewise is increased, and the filter having a length such that the unwanted mode suffers substantially a 180° differential phase shift between the two channels whereas the wanted mode suffers substantially no differential phase shift.